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A novel monohedral permanent magnet (PM) configuration is suggested and geometrically optimized to have higher magnetic efficiency as well as higher degree of access to imaging region of a magnetic resonance imaging (MRI) system. The configuration has a pair of main PMs magnetized in the opposite directions, respectively. The geometry of the main PMs is optimized by using genetic algorithm combined with a novel flexible magnetic field analysis method which combines finite-element method and new-type equivalent source method. Through an optimal design, the suggested PM configuration is proven to provide a sufficiently homogeneous and strong magnetic field for MRI application. Index Terms--Finite element method (FEM), magnetic resonance imaging (MRI), monohedral permanent magnet (PM), new-type equivalent source method (NESM).

In magnetic resonance imaging (MRI) magnets, field shimming technique is, in general, necessary to increase the field homogeneity in imaging region to an acceptable level after manufacturing. This paper presents a novel passive shimming method using permanently magnetized shimming pieces. In the method, the shimming design is modeled as an integer programming problem, and the distribution of shimming pieces is optimized. Through an application to a 0.3-T permanent magnet MRI system, the optimally designed shimming pieces are proven to improve the field homogeneity from 1456 to 784 ppm after just one iteration of shimming courses is carried out. Index Terms--Integer programming, magnetic resonance imaging (MRI), optimization, permanent magnet (PM).

A combined method is presented to deal with the practical engineering problems of multiobjective optimization. The Nondominated Sorting Genetic Algorithm II (NSGA-II) is adopted as a searching tool for the Pareto-optimal solutions, which is improved by using a new crossover operator. The response surface model (RSM) based on the radial basis function is used to reduce the computational effort. The application of the method to the shape optimization process of a permanent magnet assembly for magnetic resonance imaging devices is described, and the numerical results show that the method is feasible and efficient for the engineering shape optimization. Index Terms--Genetic algorithm, main magnet of MRI, multiobjective optimization, shape optimization.

This paper presents a new 3-D finite-element method for taking account of vector hysteresis and anisotropy. The proposed method combines the improved scalar Preisach model with the lagging angle between magnetic flux density and magnetic field intensity to solve vector hysteresis problem. Magnetic material's B - H curves with any other directions to rolling direction are obtained by interpolating the measured B - H curves to solve anisotropy problem. The method enables us to solve vector hysteresis and anisotropy problem simply. In order to validate the algorithm presented, TEAM problem32 has been analyzed using 3-D FEM as an example. Index Terms--Anisotropy, lagging angle, Preisach hysteresis model, vector hysteresis.

This paper presents a method to simulate the characteristics of permanent magnet materials with a magnetic field varying arbitrarily. Combining Preisach model and finite element analysis, a dynamic magnetic field computation is implemented for a permanent magnet assembly in a process of assembling. The hysteresis characteristic including local loop and rotational magnetization are considered in the formulation of the method proposed. The numerical results are compared with the ones of a conventional method, the electric current sheet equivalent, and the measured values to verify the validity of the presented method. Index Terms--Finite element method, magic ring, Preisach hysteresis model, rotational magnetization.